Many electrically-operated systems and devices have inductance. Some inductive devices, such as DC (direct current) motors, automotive ignition systems, and some electromagnets, can operate when supplied with a unidirectional current. Some inductive devices, such as transformers, AC (alternating current) motors, and fluorescent lights, operate when supplied with a bidirectional current.
In general, inductive elements can store energy in a magnetic field. Typically, the magnetic field is supported by an electric current flowing through the inductive element. Inductance may be a function of the characteristics of a magnetic flux path. For example, inductance in some elements may depend on material properties of a core (e.g., air, steel, and ferrite) in the flux path, and/or a flux density saturation level.
The amount of energy stored in a magnetic field of an inductive element can be a function of the inductance and the current. In general, the amount of energy stored in the magnetic field increases as current increases, and decreases as the current decreases. Accordingly, when the current through the inductive element is zero, the stored inductive energy is also zero.
One characteristic of an ideal inductor is that a voltage across the inductor is proportional to its inductance and the time rate of change of current. This concept may be represented by a formula as: V=L di/dt.
Under certain conditions, the energy stored in an inductor can generate potentially uncontrolled large voltages. This effect may be referred to by terms such as reverse electromotive force (REMF), flyback voltage, or “inductive kick.” As an illustrative example, if an inductor is being supplied a current through a switch, and that switch is rapidly opened, then the inductor may have a relatively large change of current (large di) in a relatively short period of time (small dt). As a consequence, the inductor could generate a correspondingly large voltage (large V).
In some applications, the energy stored in an inductor may be capable of generating sufficiently large voltages to damage or destroy, for example, an unprotected switch. In some systems, stored inductive energy may be dissipated as heat.